Microsoft Teams external users to the domain are unable to join via video. I have one user who has a Mac and when they try to video conference the external user will receive a message "host has blocked you from using video" or something along that line.
We don't have this issue with anyone else on Teams. She is the only one using a Mac that is the only difference.
Highly unlikely that the Mac is the issue. Sounds more like whatever account they are logged into is blocking video use via Teams in that Teams domain. Or possibly location blockage in a country that doesn't allow Teams. Although I don't think there are many places it is blocked.
Are other users from the same Login domain the user having the issue is or anything like that?
Not likely unless they have some TAP program or early release of Teams that allows meeting controls. Current production Teams only allows lobby settings essentially. They could have a meeting policy in place thou preventing video in their tenant.